The present invention relates to the heating means of a heating assembly and more particularly to a positive-temperature coefficient (abbreviated as "PTC" in this specification) thermistor device which has a high and stable thermal output and is capable of self controlling the temperature.
The PTC thermistor heating element (abbreviated as "PTCR" in this specification) has been widely used as an ideal heat source capable of providing a constant temperature in view of the facts that it has a unique characteristic of "self-temperature-control action" and that it does not react with the oxygen in the air to bring about combustion and further that it neither overheats nor generates a gas polluting the air. The equation of the steady state power output of a PTCR can be written as follows: EQU P=D(T-Ta)
in which
D=heat radiating coefficient of PTCR (W/.degree.K.) PA1 T=surface temperature of PTCR (.degree.C.) PA1 Ta=surrounding temperature (.degree.C.)
Upon being in an excited state, the PTCR has a surface temperature slightly higher than the curie point. In addition, the change in the surface temperature of PTCR is limited when an increase in the voltage is made available. Accordingly, an increase in the heat radiating coefficient D is required if an increase in the power output P is called for. For this purpose, the heat radiating means made of metal or alloy is attached to the PTCR electrode so as to permit the heat radiating coefficient D to expand. However, such technology of attaching a heat radiating means made of metal to PTCR has several limitations, which are further expounded explicitly hereinafter.
In order to expand the heat radiating coefficient D, the connecting surfaces of the heat radiating means made of metal and the PTCR electrode must be intimately coupled. In certain models of heating sets having PTCR, the heat radiating means are arranged in such a manner that they urge against the PTCR electrodes by means of biasing springs. Such heating sets are defective in that the biasing springs are subjected to thermal fatigue under high temperature, thereby resulting in a reduction in biasing force. Other models of heating sets having PTCR are designed in such a way that the metal heat radiating means are coupled with PTCR electrodes by means of an electrically conductive adhesive. Such method can effectively increase the heat radiating coefficient D but the adhesive used to connect the heat radiating means and the PTCR electrode is expensive. In addition, the association of heat radiating means with the PTCR electrode by means of such adhesive is vulnerable to collapse upon a mechanical collision or impact. A short circuit can take place when the adhesive is dripped out or squeezed out to bridge the heat radiating means and the PTCR electrode. Furthermore, such adhesive contains silver flakes, which may happen to be distributed unevenly in the adhesive. Such uneven distribution of silver flakes in the adhesive is a safety hazard because it often brings about a gap discharge and sparks. Some recent models of heating sets having PTCR are designed in such a way that the contact surface of the PTCR electrode is corrugated. In other words, the ridges of such corrugated surface of the PTCR electrode are coupled with the surface of the heat radiating means while the grooves of the corrugated surface of the PTCR electrode are filled with the electrically non-conductive adhesive, which serves to bind together the heat radiating means and the PTCR electrode. This method is also defective in that the product rejection rate of the ceramic PTCR having a corrugated surface is relatively high and that the unit contact area between the heat radiating means and the PTCR electrode is too limited to expand the heat radiating coefficient D. It must be noted here that the process of sintering a ceramic PTCR having uniformly a corrugated contact surface is often costly and time-consuming nightmare.